Synthetic stevensite and process for preparation thereof

ABSTRACT

Synthetic stevensite composed of stevensite-type sodium magnesium phylosilicate of which metallic components consist essentially of magnesium, sodium and silicon. The synthetic stevensite is characterized by having an X-ray diffraction peak at a spacing of 16 to 26 Å when treated with ethylene glycol. It can be produced by a process comprising hydrothermally treating an aqueous composition containing basic magnesium carbonate and a silica-sodium component selected from the group consisting of (i) sodium silicate, (ii) sodium silicate and amorphous silica, and (iii) amorphous silica and sodium silicate.

This is a division of application Ser. No. 07/150,613, filed on 2/1/88.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to synthetic stevensite and a process forproducing it. More specifically, it relates to synthetic stevensitewhich is formed of only three metallic components, Mg, Si and Na, hashigh purity, and is useful as a thickener, an adsorbent, etc., and to aprocess for producing it.

2. Description of the Prior Art

Synthetic hectorite has been known heretofore as a water-swellablesynthetic clay mineral, as described in Japanese Laid-Open PatentApplication Nos. 113115/1985 and 197671/1985. Synthetic hectoritecontains lithium as a monovalent metallic component and a fluorine ionas an anionic component, and because of the deleterious effect of thesecomponents on human and other creatures, is limited in use.

Stevensite is a clay mineral having a chemical composition of thefollowing formula

    (Mg.sub.2.88 Mn.sub.0.02 Fe.sub.0.02)Si.sub.4 O.sub.10 (OH).sub.2.(Ca,Mg).sub.0.07                               ( 1)

This mineral corresponds to a magnesium phyllosilicate [Mg₃ Si₄ O₁₀(OH)₂ ] in which part of the magnesium component is replaced by anothermetal and another part of it is left vacant.

Attempts have already been made to synthesize stevensite, and forexample, Clays and Clay Minerals, vol. 27, No. 4, pages 253 to 260(1979) states that hydrothermal treatment of a sepiolite-water systemyields stevensite.

As shown by the above formula (1), the naturally occurring stevensitecontains colored metallic components such as iron and manganese.Furthermore, the above known synthesis method cannot completely convertsepiolite into stevensite, and has the disadvantage that a considerableamount of sepiolite remains as an impurity in the resulting product.

In the hectorite, part of the magnesium component is replaced only by alithium atom, and therefore, the arrangement of the atoms becomesregular. Hence, hectorite is susceptible to crystallization.

On the other hand, stevensite is not regular in the arrangement of atomsand is difficult to crystallize because part of the magnesium componentis replaced by another atom and part of the remainder is left vacant.

To the best of the knowledge of the present inventors, syntheticstevensite which is composed only of the three metallic components, Mg,Si and Na, and has high purity has not yet been known.

SUMMARY OF THE INVENTION

It is an object of this invention to provide synthetic stevensite whichis composed substantially only of three metallic components Mg, Si andNa, and is free from impure metallic components.

Another object of this invention is to provide synthetic stevensitewhich has excellent whiteness and when dispersed in water or a mixtureof water and methanol or ethanol, can form a thickened solution havingexcellent transparency.

Still another object of this invention is to provide a process forproducing the above synthetic stevensite of high purity in high yields.

This invention provides synthetic stevensite composed of stevensite-typesodium magnesium phyllosilicate and having an X-ray diffraction peak ata spacing of 16 to 26 Å when treated with ethylene glycol, the metalliccomponents of said phyllosilicate consisting essentially of magnesium,sodium and silicon.

Preferred synthetic stevensite substantially has a chemical compositionrepresented by the following formula

    Mg.sub.x Na.sub.y Si.sub.4 O.sub.10 (OH).sub.2.Na.sub.z    ( 2)

wherein x is a number of at least 2 and y is a number of 0 to 0.1provided that x+y<3, and z is a number of more than 0 but not more than1.0.

According to this invention, there is also provided a process forproducing synthetic stevensite, which comprises hydrothermally treatingan aqueous composition containing a basic magnesium carbonate and asilica-sodium component selected from the group consisting of (i) sodiumsilicate, (ii) amorphous silica and sodium silicate and (iii) amorphoussilica and sodium hydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction spectral chart of the syntheticstevensite obtained in Example 1 of the invention taken by using Cu-Kαrays.

FIG. 2 is an X-ray bottom reflection spectral chart, taken by usingCu-Kα rays, of the synthetic stevensite obtained in Example 1 of theinvention and treated with ethylene glycol.

FIG. 3 is a thermal analysis curve of the synthetic stevensite obtainedin Example 2 of the invention.

FIG. 4 is a thermal analysis curve of synthetic hectorite (RAPONITE XLGmade by RAPORTE CO., LTD.)

DETAILED DESCRIPTION OF THE INVENTION

The synthetic stevensite of the invention is similar to hectorite inthat it is a magnesium silicate hydrate belonging to smectite. In thesynthetic stevensite of the invention, the alkali metal component in thelayers is Na, whereas the alkali metal component in the layers ofhectorite is Li. A further difference is that in hectorite, the totalnumber of Mg and Li atoms (x+y) in the layers is 3, whereas in thesynthetic stevensite of the invention, the total number of Mg and Naatoms (x+y) in the layers is less than 3.

The fact that the total number of Mg and Na atoms in the layers of thesynthetic stevensite of the invention is less than 3 shows that some ofMg atoms in the MgO₆ octahedral layer are replaced by Na, and some ofthe rest remain vacant. Furthermore, it is possible further that someother Mg atoms are substituted by hydrogen atoms. To compensate for thelack of the valence charges owing to the replacement of some Mg atoms byNa and to some other Mg atoms remaining vacant, Na ions exist among thestacked layers of a basic layer structure composed of an SiO₄tetrahedral layer/Mg(Na)O₆ octahedral layer/SiO₄ tetrahedral layer.

The chemical composition of formula (2) is determined as follows: Fromthe analysis of the composition of the synthetic mineral, the number ofMg atoms (x) and the number of Na atoms (y+z) per four Si atoms aredetermined. The cations of the synthetic mineral are exchanged withammonium ions, and by analyzing the composition of the ion-exchangedsynthetic mineral, the number of Na atoms (y) present in the layers isdetermined. Thus, the numbers of atoms (x, y, z) in formula (2) can bedetermined. In this case, z is the sum of the number of exchangeable Naatoms present among the layers and the number of Na atoms (α) simplyadhering to the mineral.

In the present invention, x+y is smaller than 3, and preferably notsmaller than 2. Within this condition, x is at least 2, preferably 2.6to 2.8. Furthermore, within the above condition, y is 0 to 0.1,preferably 0 to 0.05. The z value is generally greater than 0 but notmore than 1. Theoretically, the cation exchange capacity (z-α) isrepresented by the following formula.

    z-α=y+2(3-x-y)                                       (3)

wherein α represents the number of merely adhering Na atoms.

The synthetic stevensite of this invention shows an X-ray diffractionpattern inherent to a smectite clay mineral. FIG. 1 of the accompanyingdrawings is an X-ray diffraction pattern of the synthetic stevensite ofthe invention.

In the case of smectite and a smectite-containing mixed layer mineraltreated with ethylene glycol, the X-ray bottom surface reflectionappears at 16 to 26 Å. The synthetic stevensite of this invention hasthis characteristic as shown in FIG. 2.

The synthetic stevensite of this invention shows a maximum exothermicpeak at 755° to 820° C. in differential thermal analysis, whereassynthetic hectorite has a maximum exothermic peak at 700° to 750° C.FIGS. 3 and 4 are differential thermograms of the synthetic stevensiteof the invention and synthetic hectorite (RAPONITE XLG produced byRAPORTE CO., LTD.).

The synthetic stevensite of the invention is obtained in a form freefrom impure metallic components. It is generally a white powder having aHunter whiteness degree of at least 80%, preferably at least 90%.

The synthetic stevensite has a cation exchange capacity in the range ofgenerally 0.20 to 1.50 meq./g, particularly 0.2 to 1.0 meq./g. Becauseof this cation exchange capacity, the synthetic stevensite of thisinvention can be used as an ion exchange material for various cations oran ionic adsorbent for cationic substances.

The synthetic stevensite of the invention has a relatively largespecific surface area as a characteristic of a fine layer-like compound.It has a BET specific surface area of generally 200 to 500 m² /g,preferably 350 to 450 m². By utilizing this property, the syntheticstevensite of this invention can be used as an adsorbent for dyes andmalodorous components and is also expected to be used as a catalystcarrier.

Furthermore, the synthetic stevensite of the invention gets swollen withwater and alcohols such as methanol, ethanol and glycerol to givetransparent thick solutions. Since the synthetic stevensite of theinvention has no metallic components and anions which are consideredtoxic and has a high purity, it is useful as a thickening agent, aswelling agent, an emulsification stabilizer, a caking agent, a gel baseor a molding agent for cosmetics, pharmaceuticals, foodstuffs, householdgoods, agricultural goods and ceramic articles.

According to the process of this invention, the synthetic stevensite canbe produced by hydrothermally treating an aqueous mixture containingbasic magnesium carbonate and sodium silicate or a combination ofamorphous silica and sodium hydroxide.

Selection of basic magnesium carbonate as a magnesium material enablessynthesis of stevensite in high purity and yield. Any desired grade ofbasic magnesium carbonate may be used. If, however, magnesium carbonate,magnesium hydroxide, or a mixture of both is used, it is impossible toproduce stevesite of high purity in high yields. Hydromagnesite isespecially desirable as the basic magnesium carbonate. It has a chemicalcomposition of the following formula

    4MgCO.sub.3.Mg(OH).sub.2.4H.sub.2 O                        (4)

It also has an X-ray diffraction pattern assigned to ASTM No. 25-513.

An aqueous solution of sodium silicate is advantageously used as amaterial for the Si and Na components. A combination of amorphous silicaand sodium silicate and a combination of amorphous silica and sodiumhydroxide may also be used. Sodium silicate having the following formula

    nSiO.sub.2.Na.sub.2 O                                      (5)

wherein n is a number of 1 to 5, preferably 2.0 to 3.5, may be used.Amorphous silica may be, for example, silica hydrosol, silica hydrogel,silica xerogel, wet-method amorphous silica or vapor phase-methodamorphous silica.

The proportions of basic magnesium carbonate and sodium silicate or acombination of amorphous silica and sodium hydroxide may preferably besuch that the amounts of the magnesium component and the silicatecomponent are substantially stoichiometric and the amount of the sodiumcomponent is greater than the stoichiometric amount. When sodiumsilicate is used, the sodium component exists in excess in the systemwithout particularly adding sodium hydroxide.

Prior to the hydrothermal reaction, the raw materials used are mixed asuniformly as possible to form a homogeneous aqueous slurry. This isdesirable in view of the increase of yield and purity. The homogeneousmixing is preferably carried out with stirring under a strong shear. Forthis purpose, a high-speed shear mixer, a ball mill, a sand mill, acolloid mill, and ultrasonic radiation may, for example, be used.

A small amount of sodium silicate acts effectively to disperse the basicmagnesium carbonate uniformly in aqueous solution. Hence, when sodiumsilicate is used as a material, uniform mixing can be achieved by firstdispersing the basic magnesium carbonate with sodium silicate to preparea slurry, and then adding the remaining material to the slurry. Sodiumsilicate in this case may be used in an amount of 0.01 to 10% by weightbased on the aqueous slurry. Desirably, the solids concentration of theaqueous mixture is generally in the range of 1 to 30% by weight,preferably 5 to 15% by weight.

The uniform mixture is fed into an autoclave and hydrothermally treated.The hydrothermal treatment conditions may be milder than those used inthe prior art. For example, it is preferably carried out at atemperature of 100° to 300° C., more preferably 150° to 200° C., under apressure of 0 to 100 kg/cm² ·G, preferably 6 to 40 kg/cm² ·G. Thereaction time is generally on the order of 0.5 to 20 hours. Thesynthetic stevensite obtained by the reaction is separated from themother liquor by a solid-liquid separating procedure, washed with water,and dried to obtain a final product.

The present invention has for the first time provided syntheticstevensite composed substantially only of three metallic components, Mg,Si and Na, and being free from impure metallic components. Furthermore,according to this invention, synthetic stevensite of high purity can besynthesized in high yields under relatively mild hydrothermal reactionconditions using inexpensive easily available raw materials.

The following Examples illustrate the present invention.

The various tests in the following Examples were conducted by thefollowing methods.

1. X-ray diffractometry

X-ray diffractometry was conducted by using an X-ray diffraction deviceof Rigaku Denki Co., Ltd. (X-ray generating device 4036A1, goniometer2125D1, counter device 5071).

The diffraction conditions were as follows:

Target: Cu

Filter: Ni

Detector: SC

Voltage: 35 kVP

Current: 15 mA

Count full scale: 8000 c/s

Time constant: 1 sec.

Scanning speed: 2°/min.

Chart speed: 2 cm/min.

Radiation angle: 1°

Slit width: 0.3 mm

Glancing angle: 6°

2. X-ray diffraction of a sample treated with ethylene glycol

The sample (1.0 g) dried at 110° C. for 2 hours was taken, and 5 ml of a10% aqueous solution of ethylene glycol was added to it by a wholepipette. The mixture was well stirred by a stirrer, and then dried at60° C. for 12 hours. The dried product was crushed by an agate mortar,and the resulting powder was subjected to X-ray diffractometry under thefollowing conditions.

a. Conditions of X-ray diffraction

Target: Cu

Filter: Ni

Detector: SC

Voltage: 40 kVP

Current: 20 mA

Count full scale: 2000 c/s

Time constant: 2 sec

Scanning speed: 1°/min.

Chart speed: 1 cm/min.

Radiation angle: 1/6°

Slit width: 0.3 mm

Glancing angle: 6°

Diffraction angle range measured: 1°-9° (2θ)

The spacing (d) is calculated in accordance with the following equation(6) from the diffraction angle (2θ) determined from the mid-point of thehalf-value width.

    d=(λ/2) sin.sup.-1 (θ)                        (6)

where λ is the wavelength of X-ray which is 1.542 Å.

3. Analysis of the composition

The sample dried at 110° C. was analyzed for SiO₂ by gravimetry, for MgOby chelate titrimetry, and for Na₂ O and Li₂ O by flame photometry.

With regard to Na₂ O and Li₂ O in the layers, the exchangeability of asample fully swollen with water was determined. With regard to a sampleobtained by washing away the adhering cations with a 1N aqueous solutionof ammonium acetate, washing it and then drying it at 110°, theconcentrations of Na₂ O and Li₂ O were measured.

4. Thermal analysis

This analysis was conducted by using a differential thermal balance(standard type TG-DTA 8078G1) made by Rigaku Denki Co., Ltd. Themeasuring conditions were as follows:

Sample weight: 15 mg

DTA range: 100 μV

Temperature elevating rate: 10° C./min.

Atmosphere: air

EXAMPLE 1

To 400 ml of water were added 25.6 g of commercial basic magnesiumcarbonate (TT made by Tokuyama Soda Co., Ltd.) and 108.0 g (24 g assilica) of No. 3 sodium silicate, and they were mixed for 3 minutes in ahousehold mixer. Water was added to the mixture to adjust its totalamount to 600 ml. It was put in a 1-liter autoclave, and while it wasstirred with a stirrer, carbon dioxide gas was blown into it. When themixture lost flowability, the blowing of carbon dioxide gas was stopped.The mixture was then hydrothermally treated at 180° C. for 3 hours, andcooled gradually. The treated product was filtered, washed with water,and dried to give 36.2 g of the product.

Analysis of the composition of this product showed that x, y and z ingeneral formula (2) were as follows.

x=2.698

y=0.014

z=0.777

Samples (0.5 g) of the above dried product were respectively taken into50 ml. sample bottles and 25 ml of water, 25 ml of 80% methanol solutionand 25 ml 40% glycerol solution were respectively added. The bottleswere occasionally shaken to produce a transparent hydrogel, andalcoholic gels respectively. When these bottles were gently caused tofall down, the uniform gels in the bottles did not flow.

EXAMPLE 2

Commercial basic magnesium carbonate ("Kinboshi", a product ofKounoshima Chemical Co., Ltd.; 25.6 g) and 546.2 g (24 g as silica) ofsilica hydrogel (MIZUKASORB®, a product of Mizusawa Chemical) were putin a household mixer, and mixed for 3 minutes. To the mixture were addedan aqueous solution of sodium hydroxide (2.7 g as sodium hydroxide) andwater to adjust the amount of the solution to 800 ml. It was put in a1-liter autoclave, and with stirring, hydrothermally treated at 175° C.for 3 hours. Gases evolved during the process were occasionally removed.The autoclave was gradually cooled, and the contents were taken out,filtered, and dried to obtain 38.9 g of the product.

The composition of the product was analyzed, and x, y and z ingeneralformula (2) were found as follows.

x=2.67

y=0.01

z=0.65

As in Example 1, this product became a transparent gel in water andalcohol solutions and showed the same characteristics as described inExample 1.

EXAMPLE 3

Commercial basic magnesium carbonate (TT, a product of Tokuyama SodaCo., Ltd.; 1,077 g) (430 g as magnesia) was put in about 4 liters ofwater, and 1,186 g (264 g as silica) No. 3 sodium silicate was added.The mixture was stirred to prepare a slurry. Then, 21,068 g (697 g assilica) of silica hydrogel (MIZUKASORB® produced by Mizusawa ChemicalCo., Ltd.) was stirred and the slurry was added dropwise to it to forman aqueous slurry.

The aqueous slurry was put in a 40-liter autoclave, and with stirring,hydrothermally treated at 170° C. for 5 hours. Gases evolved during theprocess were occasionally removed. The autoclave was cooled gradually,and the contents were taken out, filtered and dried to give 1,617 g ofthe product.

Analysis of the composition of the product showed that x, y and z ingeneral formula (2) were as follows.

x=2.809

y=0.047

z=0.430

As in Example 1, this product became a transparent hydrogel in water,and showed the characteristics described in Example 1.

What is claimed is:
 1. A process for producing synthetic stevensite,which comprises hydrothermally treating a fluorine-ion-free aqueouscomposition containing as metallic components only basic magnesiumcarbonate and a silica-sodium component selected from the groupconsisting of (i) sodium silicate, (ii) sodium silicate and amorphoussilica, and (iii) amorphous silica and sodium hydroxide, the solidsconcentration of the aqueous composition being 5 to 15% by weight, thehydrothermal treatment being carried out at a temperature of 100° to300° C. under a pressure of 0 to 100 kg/cm² ·G, and the resultingsynthetic stevensite being non-toxic to humans.
 2. The process of claim1 wherein the basic magnesium carbonate is hydromagnesite.
 3. Theprocess of claim 1 wherein the magnesium component and the silicacomponent are used in substantially stoichiometric proportions.
 4. Theprocess of claim 1 wherein the hydrothermal treatment is carried out ata temperature of 150° to 200° C. under a pressure of 6 to 40 kg/cm².